Unitised curtain walling - how to apply modern methods of construction to the building envelope

2009-01-23

Ian Smith, Senior Manager for Wicona Projects, part of Hydro Building Systems – Europe’s largest supplier of aluminium building systems, discusses the increased demand for unitised curtain walling, the advantages of the approach and offers some practical advice to architects and contractors considering using this innovative approach to façade design.

Ian Smith, Senior Manager for Wicona Projects, part of Hydro Building Systems – Europe’s largest supplier of aluminium building systems, discusses the increased demand for unitised curtain walling, the advantages of the approach and offers some practical advice to architects and contractors considering using this innovative approach to façade design.

Off-site construction is revolutionising the way buildings are built. More developers, architects and contractors than ever before are recognising the benefits of moving elements of the construction process into a controlled factory environment.  The build process can be speeded up, quality is improved and work on site is significantly safer.

An Off-site Solution for Façade Construction

Unitised curtain walling is designed to maximise the benefits of building off site and allow modern methods of construction to be applied to the external envelope.  Whilst the approach is still relatively new to the UK, it has been used extensively in Europe and internationally.

The Burj in Dubai will be the world’s tallest building when it is completed towards the end of 2009.  This is an iconic super tower, spanning 189 floors and more than 900m high.  And it is using 120,000sqm of unitised curtain walling.

How the Approach Works

Unitised façades consist of prefabricated units, which are typically the width of a glazing bay and one or two storeys high.  The panels are fully glazed and sealed off site, with gaskets and glazing beads pre-installed in factory-controlled conditions instead of on site.  The complete units are then moved to site for fast and easy installation onto pre-prepared fixings to either steel or concrete frames.

The approach can be used for technically challenging projects, as well as fast track schemes, new build and refurbishment, and new designs can be developed for more complex and bespoke façades.

Unitised façades do not dictate uniformity – in fact an infinite range of design variations is possible within the façade structure, with a mix of panel sizes, colours and materials.

Conventional cladding can also be eliminated because the unitised panels can hold composite stone and cedar, for example, as well as glass.  This gives a single point of responsibility for the building envelope, which is easier to manage, and there are no difficult interfaces between different cladding systems.

The Advantages of a Unitised Approach

So what are the advantages of this alternative solution to façade design and construction?

• Shorter programme times.  Unitised solutions will help contractors and architects meet the increasing demand for shorter programme times, and can achieve impressive reductions in time on site of up to 70 per cent.
• Quality improvement.  Quality is significantly improved because the façade panels are completed off site, and completion of the external envelope is less affected by inclement weather.
• Aesthetics.  Unitised façades offer a wide variety of aesthetic variations allowing architects to use any combination of profiles, infills, external feature caps and glazing specification.  Aesthetic variations include stone cladding; metal, composite or polycarbonate panels; windows, doors and fixed light glazing; brise soleil sunshading, and photovoltaic panels.
• Safer work on site.  The units are installed using a platform or hoist, reducing the need for scaffolding and for work at height, improving site safety.
• Faster return on investment.  Earlier fitting out is possible, as the building envelope for each floor is completed – leading to earlier occupation and a faster return on investment for the construction client.
• Cost savings.  There are cost savings on preliminaries and scaffolding, and there is the option of standardisation to achieve economies of scale and material optimisation.
• Reduced storage and handling. The storage of cladding materials and glass handling on site can be completely eliminated – a major advantage for constrained city centre sites and where access is restricted.
• Less wastage.  There is more efficient control of materials in a factory environment compared to a building site that is exposed to the elements and with no protected and secure storage space.  The result is reduced wastage, loss and damage.
  

 
 
Key Design Considerations

The important design considerations for unitised façades are similar to those for conventional façade design, but because unitised solutions are faster on site, earlier specification decisions are required.

Design considerations include:

• Thermal performance.  Meeting the green agenda and designing façades to reduce energy consumption has to be a priority. The façade should also meet the building’s overall required U value with a combination of glass specification, infill and section design.
• Acoustics.  The impact of acoustics floor to floor – and for rooms which are adjacent on the elevation, should be considered.  Acoustic breaks can be incorporated in the façade where glazing meets partition walls, and the glass specification should be enhanced where external sound reduction is required.
• Appearance.  The façade has a huge impact on the marketability of a building – whether it is for commercial office space or apartments for sale.  Is grid or structural glazing required? – both are possible with unitised façades.  And consider the colour of finishes, sight lines, infills, spans and whether the building will be required to complement surrounding architecture.
• Fire.  Consider the specification requirements for preventing spread of fire.  This is particularly important for apartments, and can form part of the façade package, either floor slab to façade, and between adjacent dwellings
• Cost implications.  Cost always influences how a building will look and its market appeal – and there are cost differences between standard beaded and structurally glazed façades.  And whilst prefabricated glazing systems are currently more expensive, the reality is that there is less work on site so unitised solutions should be more cost effective.  This is the case in countries such as Germany where unitised façades are more widely used.
• Comfort considerations.  Specifiers need to look at how to optimise natural light to reduce the reliance on artificial lighting – and then how to address solar gain, either through the glass specification or the use of integral blinds or solar shading.  Natural ventilation is also very important – is trickle ventilation sufficient or should mechanically operated or automated vents be incorporated?  And are there external noise implications of natural ventilation?
• Maintenance.  How will the building be cleaned?  This is an issue if the building is high rise. In the case of apartments, should the windows open internally to allow the surrounding glass to be cleaned safely?  And how will glass be replaced – can it be replaced from the inside?
• Structural implications.  What will the façade be required to achieve structurally?  Look at floor spans and panel widths, openings, barrier loadings for impact resistance, wind loadings and structural movement – unitised façades are more adaptable to building movement.  How will the façade be fixed?  This will affect the design of floors and ceilings.
• Sustainability.  Consider how the façade fits into the building’s sustainability strategy.  The façade should be as energy efficient as possible and specifiers should use products such as insulated panels and high performance glass, to their best advantage.  And consider the lifespan of the material.  Aluminium is 98 per cent recoverable, and the process of re-smelting recycled aluminium uses only 5 per cent of the energy needed for new metal – making it a highly sustainable building material.

Working with Unitised Façades – Some Hints and Tips

The Design Process

Working with unitised façades is very different to conventional ‘stick’ curtain walling and requires a change of approach.  For a successful project, which maximises the benefits of unitised curtain walling, it is critical that a building is designed around the façade and not vice versa.

The façade specialist should be involved at the earliest stage to design the building structure that will best accommodate the unitised façade, the required loadings and its allowable tolerances.

And the project ideally needs to be medium to large in scale for maximum cost efficiency.  Façade projects in excess of 3,000sqm will achieve the best economies of scale, but smaller fast track schemes or developments on constrained city centre sites where access is restricted, can also be accommodated with a unitised system.

Selecting the Façade Partner

As with all façade projects, when selecting the system supplier, the specifier should look at the manufacturer’s and fabricator’s track record, project portfolio, financial stability, R&D resources, in-house facilities, technical back-up and design experience.  The façade specialist should be willing to work as an integral part of the design team from the earliest stages and participate in design workshops.

Consider whether the façade specialist will offer a complete turnkey service.  One single point of contact will give the benefit of a seamless approach, one point of responsibility and complete peace of mind from design to fabrication, site installation, testing and handover.

A team should already be in place for the façade design, fabrication and installation.  And the specifier should always look for independent approvals for the system and ask to see certificates and test results for verification.

Ensure façade systems are like for like when making comparisons and that the quality of components, technical competence of the team and best value are assessed.  Use the façade specialist for advice and involve their team from the outset to make best use of the expertise available.

Design Detailing

The design concept should be suitable for unitisation.  Flat or facetted, non-projecting façades without overhangs will allow the units to be hoisted into position for fast and efficient installation.

The structural integrity and built-in tolerances of the building frame are important to the success of the façade construction and should be specified at an early stage.

Consider what the unitised panels will be fixed to at the design concept stage – steel or concrete? The fixings will also affect the design of floors and ceilings.  What will the floor thicknesses be and will there be any possible deflections?

Managing the Project

The true façade cost should be considered, including the advantages of reduced site time, eliminating the need for scaffolding and earlier fitting out, completion and occupation, when comparing the cost of a unitised façade against traditional curtain walling systems.

Lastly, ensure the project is closely monitored and evaluated. Was it delivered on time and to budget?  If not, why not?  Use this information and the lessons learnt for setting the specification for your next façade project.

What Next?

The biggest challenge for specialists in unitised façades is to overcome the resistance to change among many building designers and contractors, and the misconception that unitised solutions have to be more expensive.

There has been a lack of innovation in façade design in the UK – but if aesthetics are balanced with cost, an innovative, bespoke and sustainable design can be produced to a budget.

And if all the benefits of unitised solutions are to be realised, façade fabricators will have to take a more manufacturing approach to the building envelope, whilst ensuring they have the production and storage capacity in place that off-site construction demands.